Trammell Brian C, Ma Lianjia, Luo Hao, Hillmyer Marc A, Carr Peter W
Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA.
J Chromatogr A. 2004 Dec 10;1060(1-2):61-76.
The synthesis and chromatographic characterization of a highly crosslinked self-assembled monolayer (SAM) stationary phase whose acid and thermal stability were significantly improved relative to a sterically protected octadecylsilane (ODS) stationary phase were recently described [B.C. Trammell, L. Ma, H. Luo, D. Jin, M.A. Hillmyer, P.W. Carr, Anal. Chem. 74 (2002) 4634]. Unfortunately, this highly crosslinked SAM phase is much more silanophilic than a conventional sterically protected octadecyl silane phase. 29Si CP-MAS NMR analysis shows that the high concentration of silanol groups in the self-assembled monolayer causes the increased retention and poor peak shape of basic solutes. In this work dimethyl-chloromethyl-phenylethylchlorosilane (DM-CMPES), a silane with only a single reactive silyl chloride group was tested as an alternative to chloromethyl-phenyethyltrichlorosilane (CMPES) as the basis for forming the starting phase. Most importantly this "conventional" silanization step (i.e., a non-SAM silanization) was followed by a Friedel-Crafts reaction using aluminum chloride as the catalyst and styrene heptamer as the multi-valent crosslinker to form the surface DM-CMPES groups into a network polymer which is fully confined and attached to the surface. An octyl (C8) derivative of the hypercrosslinked (HC) dimethyl-chloromethyl-phenylethyl (DM-CMPES) surface-confined stationary phase was synthesized to demonstrate the potential of a Friedel-Crafts based approach to making high efficiency, acid and thermally stable polymerized phases on silica with selectivity closer to conventional aliphatic phases. The stability of the retention factors of these phases under very aggressive conditions (5%, (v/v) trifluoroacetic acid and 150 degrees C) are compared to that of a sterically protected octadecylsilane (ODS) phase. The comparisons show that the long term stability of highly crosslinked DM-CMPES phases in acid is superior to the conventional phase. The HC-C8 phase is even more stable in acid than the HC-styrene heptamer DM-CMPES phase on which it is based. Additionally, the efficiency and peak shape of several prototypical bases under acidic (0.1% TFA, pH 2.0) elution conditions are discussed. The column dynamics and thermodynamic characteristics of the HC-C8 phase were investigated to demonstrate the chromatographic utility of this ultra-stable phase. Inverse size exclusion chromatography and flow studies of the HC-C8 and the sterically protected C18 stationary phases indicate the absence of pore plugging and quite good (nearly 100,000 plates/m) chromatographic efficiency. Further chromatographic investigations show that the HC-C8 stationary phase behaves as a typical reversed phase material. The HC-C8 stationary phase offers unique chromatographic selectivity for certain classes of analytes compared to both alkyl and phenyl bonded phases.
最近报道了一种高度交联的自组装单分子层（SAM）固定相的合成及色谱表征，相对于空间位阻保护的十八烷基硅烷（ODS）固定相，其酸稳定性和热稳定性有显著提高[B.C. 特拉梅尔，L. 马，H. 罗，D. 金，M.A. 希尔迈尔，P.W. 卡尔，《分析化学》74 (2002) 4634]。不幸的是，这种高度交联的SAM相比传统的空间位阻保护十八烷基硅烷相更亲硅醇基。29Si CP - MAS NMR分析表明，自组装单分子层中高浓度的硅醇基团导致碱性溶质的保留增加和峰形不佳。在本工作中，测试了仅具有单个反应性甲硅烷基氯基团的硅烷二甲基 - 氯甲基 - 苯基乙基氯硅烷（DM - CMPES），以替代氯甲基 - 苯基乙基三氯硅烷（CMPES）作为形成起始相的基础。最重要的是，在这个“传统”的硅烷化步骤（即非SAM硅烷化）之后，使用氯化铝作为催化剂，苯乙烯七聚体作为多价交联剂进行傅克反应，将表面的DM - CMPES基团形成一个完全受限并附着在表面的网络聚合物。合成了超交联（HC）二甲基 - 氯甲基 - 苯基乙基（DM - CMPES）表面受限固定相的辛基（C8）衍生物，以证明基于傅克反应的方法在二氧化硅上制备高效、耐酸和热稳定的聚合相的潜力，其选择性更接近传统脂肪族相。将这些相在非常苛刻的条件（5%，（v/v）三氟乙酸和150℃）下保留因子的稳定性与空间位阻保护的十八烷基硅烷（ODS）相进行了比较。比较结果表明，高度交联的DM - CMPES相在酸性条件下的长期稳定性优于传统相。HC - C8相在酸性条件下甚至比其基于的HC - 苯乙烯七聚体DM - CMPES相更稳定。此外，还讨论了在酸性（0.1% TFA，pH 2.0）洗脱条件下几种典型碱的效率和峰形。研究了HC - C8相的柱动力学和热力学特性，以证明这种超稳定相的色谱实用性。对HC - C8和空间位阻保护的C18固定相进行的反相尺寸排阻色谱和流动研究表明，不存在孔堵塞现象，并且具有相当好的（近100,000 塔板/米）色谱效率。进一步的色谱研究表明，HC - C8固定相表现为典型的反相材料。与烷基键合相和苯基键合相相比，HC - C8固定相对某些类别的分析物具有独特的色谱选择性。
